The AA8V 6146B Amplifier
by Greg Latta, AA8V

Typical Operating Conditions

Screen Current Limit - Important!
Because the screen voltage in this amplifier is regulated at 180V, it is very easy to exceed the maximum screen input of 3 watts and damage the tube. Under no circumstances should the screen current exceed 17 mA! (It is very easy to exceed the maximum screen current during tune-up.)

In this amplifier the dropping resistor in the screen voltage regulator circuit has been is carefully chosen to prevent the screen current from exceeding the maximum value. However, if the dropping resistor is too small, it may be possible to exceed the 17 mA limit and damage the tube. Carefully read the section on the screen voltage regulator circuit to see how to select the proper value for the screen regulator dropping resistor.

Operating Bias:
Examination of the 6146B tube data sheets shows that for linear operation the tube bias can be set for a zero signal plate current of 12 to 25 mA. Higher idle currents yield better linearity at the expense of lower power output and higher tube dissipation. Since this amplifier would be used primarily for CW operation, where linearity was not a major consideration, a zero signal plate current of 15 mA was selected.

To adjust the bias, the bias control is first set to apply maximum bias to the tube. This prevents damage to the tube when plate voltage is first applied. The amplifier is then set to "Operate" and the bias control is adjusted so that the plate current is 15 mA when no signal is applied to the input. This adjustment may have to repeated as the tube ages.

Maximum Power Output of 62 Watts:
With the operating bias set as indicated above, and driven by the 6CL6 One Tube Transmitter, the 6146B amplifier can produce an output of 62 watts on 40m as measured with a Bird wattmeter. This is consistent with the 6146B data sheets.

Conservative Output of 50 Watts:
Although the amplifier can produce 62 watts of output, the amplifier must be very carefully tuned to achieve this maximum. It turns out that cutting the drive back and limiting the output to 50 watts (a insignificant decrease of 0.8 dB) substantially reduces all operating currents and results in a cooler and thus happier amplifier. For this reason I usually run the amplifier at a conservative output of 50 watts, rather than 62 watts.

Summary of Typical Operating Conditions:
The first table below summarizes the operation of the amplifier at the maximum output of 62 watts on the 40m band. Operation on the other bands yields similar values. The second table shows typical operating conditions at an output of 50 watts.

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If you have any questions or comments, you can send E-Mail to Dr. Greg Latta at glatta@frostburg.edu